Replace many_for_each_mut with iter_many_mut.

[tim-blackbird]

Huh, that's wierd. How did I manage to close this with a force push. and why did the force push yeet everything...
:l

Objective

Replace many_for_each_mut with iter_many_mut using the same tricks to avoid aliased mutability that iter_combinations_mut uses.

Based on #5170.
View the changes for this PR here.

Why

many_for_each_mut is worse for a few reasons:

1. The closure prevents the use of continue, break, and return behaves like a limited continue.
2. rustfmt will crumple it and double the indentation when the line gets too long.

   query.many_for_each_mut(
       &entity_list,
       |(mut transform, velocity, mut component_c)| {
           // Double trouble.
       },
   );

3. It is more surprising to have many_for_each_mut as a mutable counterpart to iter_many than iter_many_mut.
4. It required a separate unsafe fn; more duplicate unsafe code to mess up.
5. The iter_many_mut API matches the existing iter_combinations_mut API.

[tim-blackbird]

I added #[inline(always)] to all next/fetch_next methods. Should I have?

[tim-blackbird]

GitHub is now refusing to update this PR with changes I'm making to this branch. rip.